Recently, as a catalyst for purifying NOx in an exhaust gas of an oxygen-excessive lean atmosphere, an NOx sorption-and-reduction type catalyst has been utilized. This NOx sorption-and-reduction type catalyst, for example, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,625, is one in which a noble metal, such as Pt and Rh, and an NOx sorption member, such as K and Ba, are loaded on a porous support, such as Al2O3. By using this NOx sorption-and-reduction type catalyst and controlling an air-fuel ratio from a lean side to a stoichiometric as well as rich side in a pulsating manner (rich spiking), since an exhaust gas, too, becomes from a lean atmosphere to a stoichiometric as well as rich atmosphere, NOx are sorbed in the NOx sorption member on the lean side, and they are released on the stoichiometric or rich side to react with reductive components, such as HC and CO, so that they are purified. Therefore, even when it is an exhaust gas from a lean burn engine, it is possible to purify NOx with good efficiency. Moreover, since HC and CO in the exhaust gas are oxidized by the noble metal and, at the same time, are consumed in the reduction of NOx, HC as well as Co are also purified with good efficiency.
For example, in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, a catalyst for purifying an exhaust gas is proposed in which an alkaline-earth metal, such as Ba, and Pt are loaded on a porous oxide support, such as Al2O3. Further, in Japanese Unexamined Patent Publication (KOKAI) No. 6-31,139, a catalyst for purifying an exhaust gas is proposed in which an alkali metal, such as K, and Pt are loaded on a porous oxide support, such as Al2O3. Furthermore, in Japanese Unexamined Patent Publication (KOKAI) No. 5-168,860, a catalyst for purifying an exhaust gas is proposed in which a rare-earth element, such as La, and Pt are loaded on a porous oxide support, such as Al2O3.
It has been found out that the purifying reaction of NOx in the NOx sorption-and-reduction type catalyst comprises a first step, in which NO in an exhaust gas is oxidized to make it into NOx, a second step, in which NOx are sorbed onto an NOx sorption member, and a third step, in which NOx released from the NOx sorption member are reduced on the catalyst.
However, in the conventional NOx sorption-and-reduction type catalyst, the granular growth of Pt occurs in a lean atmosphere, there is a problem in that the reactivities of the aforementioned first step and second step are lowered by the decrement in the catalytic active sites.
While, as a catalytic noble metal which is less likely cause such a granular growth in a lean atmosphere, Rh has been known, however, the oxidizing ability does not come up to Pt. Hence, it is thought of using Pt and Rh together.
However, when Pt and Rh are used together, there is a drawback in that the oxidizing ability of Pt lowers. Accordingly, as the addition amount of Rh increases, the reactivity of the first step, in which NO is oxidized to make it into NOx, lowers so that the sorbing ability of NOx in the second step lowers as well. Moreover, Rh is poor in terms of the compatibility to an NOx sorption member, and there is a problem in that, when Rh and an NOx sorption member coexist, the characteristics of the NOx sorption ability and Rh cannot be fully exhibited.
Moreover, in an exhaust gas, SO2, which is generated by burning sulfur (S) contained in a fuel, is included, and it is oxidized by a noble metal in an exhaust gas of a lean atmosphere so that it is turned into SO3. Then, it has become apparent that it is readily turned into sulfuric acid by water vapor, which is also contained in the exhaust gas, that these react with the NOx sorption member to generate sulfites and sulfates, and that the NOx sorption member is thereby poisoned to degrade. This is referred to as the sulfur poisoning of the NOx sorption member. In addition, since the porous oxide support, such as Al2O3, has a property that it is likely to adsorb SOx, there is a problem in that the aforementioned sulfur poisoning has been facilitated. Then, when the NOx sorption member is thus turned into sulfites and sulfates so that it is poisoned to degrade, it can no longer sorb NOx, as a result, there is a drawback in that the NOx purifying performance lowers after a high-temperature durability test (hereinafter, referred to as “post-durability”).
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 10-356, there is disclosed an NOx sorption-and-reduction type catalyst which is made by intermingling a first powder, in which Rh is loaded on Al2O3 or ZrO2, and a second powder, in which an NOx sorption member and Pt are loaded on Al2O3. In accordance with this NOx sorption-and-reduction type catalyst, since Rh and Pt are loaded separately, the oxidizing ability of Pt is inhibited from lowering. Moreover, since Rh and the NOx sorption member are loaded separately, the poorness in the mutual compatibility is not revealed so that the characteristics of the NOx sorption member and Rh are fully exhibited.
Moreover, in accordance with ZrO2 with Rh loaded (hereinafter referred to as an “Rh/ZrO2 catalyst”), hydrogen having a high reducing power is formed out of HC and H2O in an exhaust gas (a steam reforming reaction), and this hydrogen contributes to the reduction of NOx and the elimination of SOx from sulfates of the NOx sorption member. Thus, the NOx reduction ability in rich spiking becomes high, and the sulfur poisoning becomes remarkably less.
However, compared with Al2O3 which is often used as a support for noble metals, ZrO2 exhibits lower heat resistance, the specific surface area is decreased by the heat in service, and thereby there is a drawback in that the dispersibility of Rh lowers so that the hydrogen generating ability lowers. Moreover, since ZrO2 is a basic support, SOx in an exhaust gas are likely to approach, and accordingly a phenomenon arises in which the hydrogen generating ability is lowered by the sulfur poisoning of loaded Rh.
Accordingly, in an NOx sorption-and-reduction type catalyst including an Rh/ZrO2 catalyst, there is a problem in that the post-durability NOx purifying ability is lowered by the lowering of the hydrogen generating ability of the Rh/ZrO2 catalyst.
Moreover, it has been understood that another cause, by which the post-durability NOx purifying ability is lowered, lies in that a reaction occurs between cordierite, which has been used as a support substrate in general, and an alkali component, which works as the NOx sorption member, and thereby the NOx sorption ability has been lowered.
Namely, the alkali component is likely to move in a coating layer, such as Al2O3, and simultaneously exhibits a high reactivity to Si. Accordingly, it is believed that the alkali component moves in the coating layer when the temperature is high, arrives at the support substrate and reacts with Si in cordierite to form silicate, being a composite oxide. Since this silicate is a stable compound, the alkali component, which has been turned into silicate, no longer has an ability of sorbing NOx, and accordingly the NOx purifying performance has been lowered.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 8-117,602, an NOx sorption-and-reduction type catalyst is proposed in which a Ti—Zr composite oxide, an NOx sorption member and a noble metal are loaded on an Al2O3 support.
By thus making a composite support in which a Ti—Zr composite oxide is loaded on Al2O3, it is possible to highly maintain the initial NOx purifying rate due to the advantage of Al2O3. Moreover, by loading a Ti—Zr composite oxide, it is possible to raise the acidity of the support itself. Therefore, since this composite support, compared with the case where Al2O3 is used alone, is less likely to adsorb SOx, and since adsorbed SOx are likely to eliminate at a low temperature, the contacting probability between the NOx sorption member and SOx is lowered In addition, in a support in which a Ti—Zr composite oxide is loaded, it is believed that the movement of alkali component is suppressed, and it has been understood that the reaction of the alkali component to TiO2 is suppressed. Therefore, when the aforementioned composite support is used, a high NOx purifying rate is secured even in the initial period, since the sulfur poisoning as well as the solving of the alkali metal into the support are suppressed, a post-durability NOx purifying rate is improved.
However, in a catalyst in which a Ti—Zr composite oxide is loaded on an Al2O3 support, compared with a composite support comprising TiO2—Al2O3 as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 8-099,034 or a catalyst which uses a support in which TiO2 is added to Al2O3, it is not possible to say that the suppression effect of the sulfur poisoning is sufficient, and accordingly it has been required to further suppress the sulfur poisoning and improve the NOx purifying rate.
Moreover, due to the recent improvement of engine performance, the increase of high speed driving, and the like, the temperature of exhaust gas has risen so that the improvement of NOx purifying activity in a high temperature region has become an assignment. This is because, in a high temperature region, NOx, which have been sorbed in an NOx sorption member, are likely to be released, and the NOx sorption amount is likely to be insufficient in a lean atmosphere. Hence, K, which exhibits a high basicity and is stable at a high temperature, is made into an NOx sorption member, but, in a catalyst in which K is loaded on Al2O3, and so on, there has been a problem in that the NOx sorption ability in a high temperature region is not improved so much.
Hence, it was thought of using a support which exhibits a higher basicity, and, in Japanese Unexamined Patent Publication (KOKAI) No. 5-195,755, a catalyst is disclosed in which K and a noble metal are loaded on ZrO2. In accordance with this catalyst, since the basicities of the support and NOx sorption member are high, it is likely to sorb but less likely to release NOx. Therefore, the NOx sorption ability in a high temperature region is improved, as a result, the NOx purifying performance in a high temperature region is improved.
However, in the catalyst in which K and a noble metal are loaded on ZrO2, since the basicity of the support is high, there has been a drawback in that it is likely to sorb not only NOx but also SOx so that the lowering of the activity by the sulfur poisoning is considerable. Namely, when the basicity of a support is high, although the NOx sorption ability is improved, even the sulfur poisoning is facilitated. On the contrary, when the basicity of a support is low, although the sulfur poisoning is suppressed, even the NOx sorption ability is lowered.
The present invention has been done in view of such circumstances, it is a main object to improve the heat resistance and sulfur-poisoning resistance of an Rh/ZrO2 catalyst, being a hydrogen generating catalyst, and, being accompanied therewith, to improve the post-durability NOx purifying ability of an NOx sorption-and-reduction type catalyst.
Moreover, the second object of the present invention is to further suppress the sulfur poisoning of an NOx sorption-and-reduction type catalyst, thereby furthermore improving the high-temperature post-durability NOx purifying rate.
In addition, a further object of the present invention is to make a catalyst which can satisfy both of the contradictory phenomena, such as the improvement of the NOx sorption ability in a high temperature region and the suppression of the sulfur poisoning, and which is good in terms of the heat resistance.
The characteristic of a hydrogen generating catalyst, which solves the aforementioned assignments and is set forth in claim 1, lies in that Rh is loaded on a support comprising a ZrO2—Al2O3 composite oxide including first composite particles having a structure comprising a core portion in which ZrO2 is a major component and a superficial portion formed on a surface of the core portion and including Al2O3 more than the core portion.
Al2O3 can desirably be included in an amount of from 1 to 30% by mol in the ZrO2—Al2O3 composite oxide.
Then, the characteristic of a catalyst for purifying an exhaust gas, of an NOx sorption-and-reduction type catalyst comprising a noble metal and an NOx sorption member loaded on a porous oxide support, which solves the aforementioned assignments and is set forth in claim 4, lies in that the hydrogen generating catalyst of the present invention is included. The porous oxide support can desirably include second composite particles having a structure comprising a core portion in which ZrO2 is a major component and a superficial portion formed on a surface of the core portion and including an oxide, whose basicity is lower than ZrO2, more than the core portion.